Norovirus: culprit for gastroenteritis in NZ and worldwide

The widely reported gastroenteritis outbreak in NZ and Australia in 2013-14 has been traced to a single strain of norovirus that emerged in 2012 in Sydney.

Norovirus is a ubiquitous virus found throughout low to high income countries. The virus is incredibly common, causing vomiting and diarrhea especially in young children from low income countries. Norovirus is responsible for over 200,000 deaths and carries a $60 billion burden each year globally.

“Norovirus is the most common cause of diarrheal episodes globally, the principal cause of foodborne disease outbreaks in the United States, a key health care–acquired infection, a common cause of travel-associated diarrhea, and a bane for deployed military troops.” Benjamin A. Lopman et al.

Currently there is no specific treatment available for norovirus, other than supportive treatment for the resulting dehydration. Symptoms in otherwise healthy adults last for around 48 hours, but can be more severe in children, the elderly or immunocompromised.

This finding has been published by New Zealand and Australian scientists as part of an international collection of norovirus research released at the end of last month.

Norovirus is a single stranded RNA virus – meaning it is diverse and can rapidly evolve. This poses challenges for the development of an effective vaccine. Its ubiquitous nature indicates that improvements in water quality and hygiene are not entirely effective at combating the transmission of this extremely contagious virus.

The research collection, published by PLOS – “The Global Burden of Norovirus & Prospects for Vaccine Development” provides comprehensive analysis from norovirus experts across the globe and identifies three broad areas where there are gaps in the knowledge of this virus that hinder the development of a successful vaccine.

Measuring the disease burden

It is challenging to measure the disease burden of norovirus. The sensitive and specific diagnostics available are restricted to research and public health labs and are not for clinical use. Therefore tests cannot be carried out on all patients that present with norovirus in hospitals. In addition to this, a recent systematic review discovered that 20% of sick children were found to be infected by norovirus, while the virus was identified in 8% of children who were asymptomatic. This raises questions of when and why does the virus cause disease.

Epidemiological and economics characteristics of various age groups for considering norovirus vaccines. PLOS

Vaccine development

The collection of research also highlighted the challenge of vaccine development as norovirus cannot be grown in the lab. Exciting research shows promise for a vaccine-like particle that ‘looks’ identical to the virus and is recognised by the immune system as such, but it lacks a genome and cannot multiply.

Adult volunteer trials have produced successful results. However, trials were only carried out on healthy adults who will have been exposed to the virus previously, enabling their immune memories to be triggered. The response in children is unknown if they have not had contact with the virus before. The candidate vaccines show potential, but more research is necessary.

Vaccine implementation

The third major challenge identified by the research involved the implementation of the vaccine. Target populations will need to be strictly identified – from children to adults to the military, which will involve additional cost. The availability of the vaccine must also be carefully considered.

Ultimately there is still a lot to learn about norovirus. There is an impetus for well-conducted studies in low and middle income countries where the disease burden is highest and data is scant. Better understanding of how the virus is spread within communities is also required across all age groups.

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